Error Messages:
Cannot copy document: The specified network name is no longer available
The semaphore timeout period has expired
File creation error - The specified network name is no longer available
Error 64
I've see two cases now where people complain that their computers lose connections. They are transferring files or running a remote backup and all of a sudden they get an error about the file not existing or the drive not available.
In both cases it tuned out to be a bad network card. So make sure you check that if you are having this problem.
More Info: I'm also told now that this can be caused by a Trust between a Unix resource domain and an NT domain if there is a pass through authentication problem. I wasn't able to get more detail than that but the user said when the auth problem was fixed the error 64 went away as well.